I switched to the CGMZ party plug in and your MK summon works perfect! It seems that Mk summon and visu stella party just will not work together. I changed party plug ins and everything is back to normal! Thanks!
When my plugin works with CGMZ but not with VS, it is possible that the problem lies more in VS's hands than in mine.
Temporarily bypassing the party limit (i.e. 5 actors when the limit is 4) is not well supported in RPG Maker. Yes, there is the "exceed limit?" option in MK_Summon, but I no longer work on it. Also see its description on my blog:
https://decinbr.itch.io/rpg-maker-mz-plugin-party-swap-css-style
This plug in works with your MK summon and it has a party scene like visu stella. I would recommend this to others having the same problem as myself. It p robably just saved my game. I can now use both plug ins and return to my work. Thanks.